Magnetic Resonance Angiography (MRA) uses Magnetic Resonance Imaging (MRI) to image blood vessels. For example, MRA may be used to generate images of arteries in order to detect different conditions, such as stenosis, occlusions or aneurysms. MRA may be used to image arteries in different regions of a patient, including the head and neck arteries, the thoracic or abdominal aorta, the renal arteries, and the peripheral arteries, to depict the vascular anatomy or to provide functional information regarding blood flow. MRA can also be used to generate images of the veins which typically run in parallel, but with blood flow in the opposite direction to arteries.
There are different methods for generating MRA images. For example, contrast enhanced MRA (CE-MRA) includes injecting contrast agents into a patient and imaging the contrast agent as the contrast agent passes through the arteries or veins of the patient. However, in CE-MRA, timing is important such that any timing problems can result in a degradation of image quality. Additionally, the exogenous contrast agent may pose a contraindication for patients with renal functional impairments because of the poor clearance of the contrast agent from the body. One such risk was identified with chelates of Gadolinium used as a contrast agent where a patient with severely compromised renal function exhibited symptoms of Nephrogenic Systemic Fibrosis (NSF).
Phase-contrast MRA (PC-MRA) is also known and used to encode the velocity of moving blood (of both arteries and veins) in the phase component of the MR signals. However, PC-MRA often has a limited image field-of-view and, when triggered using electro-cardiogram (ECG) signals, provides aggregate blood flow velocity information averaged for a single cardiac cycle.